1. Field of the Invention
This invention relates to a trimming element used for, for example, microcircuits such as thin-film or thick-film integrated circuits, and more particularly to a trimming element used for short-circuiting part of a network such as active circuit elements (transistors and the like) and passive circuit elements (resistors, capacitors, inductors and the like) in redundancy processing or a functional trimming.
2. Description of the Related Art
Recently, in the monolithic semiconductor integrated circuit and hybrid integrated circuit, a functional trimming process has received much attention as a means of attaining output characteristics of high precision, for example, a high precision D/A conversion output characteristic in the case of a D/A converter.
The trimming process utilizing a laser beam can be effected with the trimming device electrically separated from to-be-trimmed material. Therefore, it is possible to cut or melt the to-be-trimmed element, a for example, resistor which is used as the main factor to determine the output characteristic of a circuit, by use of a laser beam while the circuit is set in the operative condition and the output characteristic is observed. Thus, the trimming operation can be effected to adjust the resistance until a desired output characteristic can be attained, thereby obtaining precision output characteristic. Such a trimming operation is called functional trimming.
Various trimming methods for adjusting the resistance have been proposed. One trimming method is to use short bars 2 respectively connected in parallel with series-connected diffusion resistor elements or thin film resistor elements 1 as shown in a circuit of FIG. 8A. Short bars 2 are sequentially cut apart (indicated by mark x) to adjust the resistance between two terminals A and B. The other trimming method is shown in FIG. 8B. FIG. 8B is a plan view of thin film resistor 4, and 3 denotes a metal electrode. With this method, the resistance can be adjusted by forming a groove in a resistance film to change the direction of electrode lines of force.
In the two trimming methods described above with reference to FIGS. 8A and 8B, the resistor is trimmed so as to increase the resistance thereof, and it is impossible to reduce the resistance. Therefore, when the trimming process is effected to change the resistance and if the resistance exceeds a desired resistance value, it becomes impossible to attain the desired resistance value.
In particular, in the short bar cutting system shown in FIG. 8A, since the resistance of the resistor may be made different according to various factors in the manufacturing process, there is a high possibility that the resultant resistance may be made larger than the desired resistance value in the resistance adjusting process. In order to solve this problem, the trimming resistor is set to an initial value which is sufficiently smaller than the desired value so that the resistance adjustment range can be set sufficiently large. Further, in this case, variation in resistance in each cutting operation is set to a small value in order to attain a fine resistance adjustment. However, in this method, the occupied area of the trimming resistor on the IC chip may increase, making this method impractical.
Further, there is provided a method in which PN junction diodes respectively connected in parallel with the resistors are used as a means for reducing the resistance of the resistor by the trimming process. FIG. 9 shows the means in the form of an electrical circuit. With this circuit, resistor 7 may be short-circuited to reduce the resultant resistance by applying a laser beam to PN junction diode 6 to thermally destroy the PN junction thereof and break the rectification characteristic thereof. However, in this method, it is necessary to provide a region which insulatively isolates the diode from the other circuit elements. As a result, the occupied area of the trimming resistor is increased and the number of manufacturing steps will increase because of addition of the insulation isolation step, thus making this method impractical.
In a semiconductor device shown by the sectional view of FIG. 10, the upper portion of a wafer is used as emitter layer 8, and the lower portion of the wafer is used as N-type collector layer 10 with P-type base layer 9 disposed therebetween. Further, a short-circuiting diode with N-type layer 11 and P-type layer 12 is formed in the same wafer. In this case, in order to insulatively isolate collector layer 10 from the short-circuiting diode, it is necessary to provide dielectric isolation layer 13 or PN isolation diffusion layer 13, causing a practical disadvantage.
When the short bar cutting method shown in FIG. 8A or groove forming method shown in FIG. 8B is used among the trimming methods for adjusting the resistance, the resistance can be adjusted only in an increasing direction. For this reason, in a case where the resultant resistance is set to within the desired resistance range, it is necessary to significantly increase the occupied area of the trimming resistor. Further, in the method in which diodes are connected in parallel with the resistors as shown in FIG. 9 and the resultant resistance can be reduced by the trimming operation, it is necessary to provide a complicated isolation structure as shown in FIG. 10 in order to attain the insulative isolation between the diode and the other circuit element.